Since long various techniques are well known for measuring the thickness of a flat mail item on the fly in a postal processing machine such as a postage meter, a sorter or a folder-inserter. However, thickness measurement remains generally hard to perform with a simple and economical solution as filled mail items do not generally present a uniform thickness on the one hand, and jolts and rebounds to which these mail items are subject, distort this determination on the other hand. One can cite the conventional electromechanical technique, the laser reflection technique and the imaging technique which are illustrated e.g. in the following prior art references.
U.S. Pat. No. 7,182,339 discloses a new and improved thickness measuring device in which a rotary encoder is mounted upon a mounting bracket, and a movable lever arm is fixedly connected to the rotary shaft of the rotary encoder. A first end of the lever arm has an idler wheel or roller mounted thereon which is adapted to be disposed either in contact with an outer one of a pair of conveyor belts which are provided for conveying mail pieces or units along a conveyor path defined between the pair of conveyor belts, or alternatively, in contact with a single conveyor belt along which the mail pieces or units are conveyed. A hydraulic damper is disposed in contact with the first end of the lever arm, while a second opposite end of the lever arm is fixedly connected to a biasing return spring.
U.S. Pat. No. 7,151,608 relates to a method of using reflection of a laser ray for measuring the thickness of a flat mail item moving through a sorting machine. This method consists in nipping the mail item by means of an elastically deformable member that is movably mounted and that moves with the movement of the mail item, said member having a first surface in contact with one face of said mail item, and a reflective second surface substantially parallel to said first surface; and directing the laser ray onto said reflective second surface for the purpose of measuring the thickness of the mail item.
EP 1 764 747 describes an improved apparatus and method for determining the thickness of a mail piece for dimension-based postal-rating schemes. This includes capturing a first image of at least a portion of a surface of a mail piece, determining a first optical image for a feature from the first image, capturing a second image of the portion of the surface of the mail piece after the mail piece has been transported some distance, and determining a second optical image for the feature from the second image, the second optical image forming an angle with respect to the first optical image. The first image and the second image are then correlated to determine a displacement of the feature, and thickness of the mail piece can then determined based on the displacement of the feature and the angle between the first and second optical axes.
Though equipments and methods mentioned above have provided satisfactory services or results in the past, they did not always provide a simple and inexpensive way to monitor measurement quality in real-time.